VPS34 - autophagy initiator and regulator of endosomal trafficking

Mon, 08/31/2015 - 15:15

VPS34, vacuolar protein sorting 34, is the only identified Class III phosphoinositide-3 kinase (PI3K) in mammals and is ubiquitously expressed in all eukaryotic cells. VPS34 is a 100 kDa protein responsible for phosphorylating phosphatidylinositol to produce phosphatidylinositol 3-phosphate (PI3P). PI3P is an important intermediate in the development of the double-membraned autophagosome during autophagy, indicating a role for VPS34 in autophagy initiation. PI3P allows VPS34 to form complexes with ATG14L during the elongation of the autophagosome membrane. PI3P also acts as a scaffold for other proteins necessary for autophagy progression. ULK1 was recently identified as an activating kinase of the VPS34 complexes (1). ULK1 is activated by mTORC1 or AMPK during states of cell starvation. Active ULK1 phosphorylates Beclin-1, which is bound to ATG14L in the VPS34 complexes. This signaling cascade is responsible for inducing autophagy in states of nutrient deprivation. VPS34 also forms stable complexes with VPS15, a protein kinase that activates the VPS34 kinase. The VPS34-VPS15 complex interacts with Rab5 which targets the complex to endosomes for retrograde trafficking to the Golgi.

Shi et. al. used the VPS34 antibody to understand how a known chemotherapy affects the interplay between autophagy and apoptosis (2). The group used the VPS34 antibody to assess levels of VPS34 during treatment with Saxifragifolin D (SD), a commonly used and approved therapy for solid tumors. The VPS34 antibody showed a significant increase in VPS34 levels during treatment with SD, indicating that SD is capable of inducing autophagy in the breast cancer cell lines they tested. The group also found other pro-autophagic proteins such as LC3-II and Beclin-1 to be upregulated during SD treatment. Interestingly, pharmacologic or siRNA inhibition of autophagy decreased the rate of apoptosis, indicating a role for autophagy in inducing apoptosis in the tumor cells.

Huang et. al. sought to understand how the PML-RARα fusion protein contributes to autophagy in leukemia cells (3). The group used the VPS34 antibody for western blot to assess the levels of autophagy in leukemia cells transfected with the PML-RARα fusion protein. The VPS34 antibody showed an increase in VPS34 levels in cells transfected with the fusion protein, suggesting that PML-RARα induces autophagy in these cells. The group also used a VPS34 inhibitor which completely prevented autophagy induction by PML-RARα. All of this evidence together suggests a direct interaction between PML-RARα and VPS34 in leukemia cells undergoing autophagy.